i know I've done it before but that was a while ago so I've lost track of it

The owner alerts, the player that wants it leaves, takes it, and then applies to legion.
There is no rule you "have" to accept him/her back though

This doesn't work , It was fixed a long time age.

The only way it to abandon it , or find someone you trust that has it scanned and ether have them come to your legion and alert it when the owner of the planets leaves or you go to the legion of the player that has it scanned.

Politely?

the only way to do it is to get them to abandon the planet, then they can alert it as uncolonised.
the only drawback with this is that it costs 5gp each time.
if a legionmate is leaving the game it may be worth sending him the keycode to a f/b credit gift card so he can distribute his best planets to the legion.